The optical density of blood corresponds to a number of factors and the measurement of the optical density of the blood has been used to determine certain blood parameters.
For example, during a hemodialysis session when fluid is being removed from the blood stream by a dialyzing machine, the concentration of hemoglobin, naturally occurring in the red blood cells, may decrease or, generally increase, with respect to the equilibrium processes of the fluid removal and mobilization from the body tissues. The change in the concentration of hemoglobin results in a corresponding change in the optical density of the blood and may be registered.
Monitoring changes in blood volume, as well as blood hemoglobin, can help to prevent such complications as hypotension due to continuous hypovolemia. Cardiac output of a patient, as well as other hemodynamic parameters of cardiovascular system, also provide useful information about the patient condition during hemodialysis, surgery, and in intensive care units. Using a dilution technique it is possible to calculate several characteristics of cardiovascular system, particularly cardiac output.
There are several research works and patents in the field of photometric blood monitoring. As shown in U.S. Pat. No. 3,830,569 to Meric; U.S. Pat. No. 4,243,883 to Schwartzmann; and U.S. Pat. No. 4,303,336 to Cullis, a device with suitable light source and photodetector may be used. Multiple detectors may be used, as shown by Meric.
As illustrated by U.S. Pat. No. 4,745,279 to Karkar et al.; U.S. Pat. No. 4,776,340 to Moran et al.; U.S. Pat. No. 5,048,524 to Bailey; and U.S. Pat. No. 5,066,859 to Karkar, additional detectors may be used to compensate various measurement artifacts, particularly for variations in intensity of light entering the blood stream.
However, the correlation between blood parameters and data obtained by photometric methods needs to be enhanced.
In U.S. Pat. No. 5,331,958 to Oppenheimer, a through photodetector and a remote photodetector are used for correction of light scattering due to sodium concentration fluctuations in the blood. Signals of the through and the remote photodetector are amplified separately. The data from the remote detector is used to compensate or adjust the data of the through detector. These and other dual correction or compensation systems are subject to the inherent unreliability of multiple detectors, as well as calibration issues.
A disadvantage of other methods of photometric analysis of blood properties, which utilize conventional external tubing, includes a dependence on the physical characteristics of the tubing such as diameter, wall thickness, and light absorption factor.
Therefore, the need exists for a method and apparatus which require only a single light detector, wherein the light scattering effect is eliminated or minimized. In such an apparatus, the mechanical construction and data collection process is substantially streamlined as compared to multiple photodetector devices. There is a need for a method and apparatus having automatic calibration of the optical probe sensitivity for each particular tubing that the probe is employed with; and further having a dilution technique applicable for determining hemodynamic parameters of a patient.